If you're looking for some stock market thrills, it's hard to do better than small-cap biotech stocks. This year didn't start out with a bang for these two biotechs, but their performances over the past few months have been breathtaking. 

Read on to see what made these tremendous gains possible.

Company Market Cap Jan. 1, 2020 Market Cap Oct. 15, 2020 Price Increase in 2020
Cardiff Oncology (CRDF -2.26%) $10.7 million $626 million 1,080%
Celldex Therapeutics (CLDX 1.14%) $37.9 million $459 million 618%

Data source: Yahoo! Finance.

1. Cardiff Oncology: Just $2 million up front

In 2017, this company licensed its lead candidate, onvansertib, from a privately held company for just $2 million up front after it failed to shrink any tumors during a phase 1 dose determination study. The first clinical trial with the molecule that has become Cardiff Oncology's lead candidate enrolled just 21 advanced-stage cancer patients with a variety of tumors that lacked effective standard-treatment options.

In September, Cardiff Oncology stock rocketed higher after reporting much better results from an interim analysis of an ongoing study with advanced-stage colon cancer patients that have KRAS-mutated tumors. Tumors driven by KRAS mutations are more aggressive than usual, so it was encouraging to see disease progression from just one out of 11 patients treated with onvansertib during the first six months of treatment.

Shares of Cardiff have soared around 1,100% in 2020 because, at the beginning of the year, it was a $10.7 million company with an oncology candidate that had been heading for the scrap heap before Cardiff scooped it up for next to nothing. Second-line treatment options for KRAS-mutated colon cancer are limited, and filling this gap with onvansertib could eventually deliver more than $1 billion in annual revenue to Cardiff Oncology's top line. 

Chart made of banknotes.

Image source: Getty Images.

2. Celldex Therapeutics: A 37-year itch

This clinical-stage biotech has been developing targeted therapies since 1983 with some success in midstage clinical trials. Unfortunately, none of Celldex Therapeutics' candidates have been successful enough support a new drug application.

Shares of Celldex soared in June when the company reported encouraging results from a phase 1 safety study with healthy patients and CDX-0159, an experimental treatment for hives that last for at least six weeks, a condition called chronic spontaneous urticaria (CSU). An intravenous infusion with CDX-0159 suppressed mast cell activity compared to patients given a placebo, which suggests the drug could reduce the formation of itchy red welts. 

An estimated 3.2 million Americans experience CSU, and standard treatment with antihistamines only works about half the time.

Time to buy?

There may be an unmet need for new CSU treatments, but it's probably a little too early to get excited about Celldex Therapeutics' lead candidate. While CDX-0159 appears to reduce mast cell activity, we don't know if this will make a difference for people who actually have the debilitating disorder. 

Celldex recently began a placebo-controlled study that will enroll around 40 patients who actually have CSU instead of healthy volunteers. It's probably better to wait for initial results from this study before risking any of your hard-earned money on this stock.

Cardiff Oncology may have found a niche for onvansertib, but impressive disease control results that set off fireworks at an oncology study's outset rarely translate to actual survival benefits. Given onvansertib's previous flop, there's a fair chance that the standard treatments Cardiff's candidate is being added to are doing all the work.